Sorbitol (D-glucitol) is an acyclic polyol found in a number of plant species (Kuo et al. (1990) Plant Physiol 93:1514-1520). Sorbitol is the primary photosynthetic product in rosaceous fruits and can account for a major portion of the carbon transported from the leaf. In corn sorbitol is found in seed and silk but not in pollen and leaf and low amounts of sorbitol are detectable in developing corn kernels. Sorbitol is found in soybeans and it is suggested that the accumulation of sorbitol may play a role in facilitating hexose metabolism in germinating seedlings (Kuo et al. (1990) Plant Physiol 93:1514-1520). During germination, soybeans convert oil and soluble oligosaccharides into sucrose which is in turn converted to glucose and fructose to fuel rapid growth. Some investigators have speculated that since plant fructokinases exhibit strong substrate inhibition by fructose, the presence of a sorbitol pathway may provide a mechanism to bypass this inhibition by converting excess fructose into sorbitol. This would help facilitate the metabolism of free glucose and fructose.
The metabolism of sorbitol has been extensively studied in several plant species (Kuo et al. (1990) Plant Physiol 93:1514-1520, Loescher (1987) Physiol. Plantarum 70:553-557, Loescher et al. in: Photoassimilate Distribution in Plants and Crops, ed. Zamski et al. Marcel Dekker, Inc., New York). There are several enzyme activities involved in sorbitol metabolism. Three of these enzymes are aldehyde reductase (NADPH-dependent aldose 6-phosphate reductase), sorbitol dehydrogenase, and NADP-dependent D-sorbitol-6-phosphate dehydrogenase. Aldehyde reductase appears responsible for the conversion of glucose to sorbitol; NADP-dependent D-sorbitol-6-phosphate dehydrogenase is also involved in sorbitol synthesis, and sorbitol dehydrogenase is involved in the conversion of sorbitol to fructose. Accordingly, the availability of nucleic acid sequences encoding all or a portion of these enzymes would facilitate studies to better understand carbohydrate metabolism and function in plants, provide genetic tools for the manipulation of the sorbitol biosynthetic pathway, and provide a means to control carbon partitioning in plant cells.